Heat exchange fluids are used in a wide variety of applications to absorb and transport heat away from heat producing surfaces and/or to provide freezing protection depending upon the application. Some examples include: internal combustion engine cooling systems, aircraft deicing, roadway deicing, fuel-cell cooling: systems, heat storage and heat sink systems, solar energy units, refrigeration systems, fire protection systems, hydraulic systems, pharmaceutical reactors, hot water heating systems, air conditioning systems, drilling fluids and power station cooling systems, to name a few. Water is the most commonly used heat exchange fluid because of its universal availability, low cost, excellent heat transfer properties and common use in most contemporary heat exchange applications. However, while water is the preferred heat exchange fluid because it exhibits an optimum combination of high specific heat, high thermal conductivity and low viscosity, it has serious disadvantages, including, metals corrosivity under normal operating conditions, a relatively high freezing point, a nearly 9% expansion upon freezing and a relatively low boiling point. In heat exchange applications these disadvantages mitigated by mixing water with a selection of additives that reduce the freezing point, raise the boiling point and reduce the metal component corrosivity of the water present in heat exchange systems.
Many different additives that function as freezing point depressants in water are known in the prior art. These include, among others: inorganic salts, petroleum products, organic hydroxyl compounds and low molecular weight organic acid salts. For example, early in the development of the internal combustion engine, coolant solutions were formulated with high concentrations of inorganic salts, such as calcium chloride, to depress the freezing point of the solution. One of the major disadvantages of such inorganic salts is the high concentrations necessary to achieve suitable freezing point protection. High concentrations of inorganic salts are extremely corrosive to cooling system components, especially the metal components. Adding to this disadvantage, these corrosive effects cannot be adequately mitigated by the addition of corrosion inhibitors. And, moreover, at very low temperatures the solubility of the inorganic salts is reduced, which further limits the freezing protection level that can be attained in aqueous heat transfer fluids.
Contemporary heat transfer fluid technology includes the use of organic hydroxy compounds, in lieu of inorganic salt compositions, for freezing protection because such compounds are safer, less corrosive and more effective freezing point depressants. In general, water and glycol mixtures are the preferred heat transfer fluid/antifreeze mixtures because such mixtures are chemically stable, compatible with the elastomers and plastics used in modern heat-exchange systems, provide cost efficient freezing and boiling protection and can be formulated with a variety of corrosion inhibitors to provide the specific corrosion protection required for particular heat exchange systems. Compared to water, glycols have a high specific heat, low thermal conductivity and high viscosity. Thus, when glycols are mixed with water the resulting aqueous glycol solutions, as compared to pure water have higher viscosities, higher specific heat, lower thermal conductivity and a lower heat-exchange capacity. However the benefits of freezing point reduction outweigh the loss in heat exchange efficiency.
Among the glycols, ethylene glycol is preferred as a freezing point depressant because of its high boiling and flash points compared to, for example methyl alcohol, its lower viscosity (better fluidity) and relatively lower cost. The primary disadvantage of ethylene glycol is toxicity to animals and other adverse environmental consequences that may result from the improper disposal or other releases into the environment. Other compounds similar to ethylene glycol that are in limited use include glycerol, the lower boiling alcohols, such as methanol and propylene glycol. These compounds are usually selected to mitigate the potential toxicity and possible adverse environmental consequences associated with ethylene glycol.
To address the environmental contamination and toxicity concerns a variety of non-glycol based heat transfer fluid/coolant solutions are being developed. Included among these are the alkali salts of low molecular weight organic acids such, as alkali metal acetate and alkali metal formate, which, like glycol, provide frost protection when dissolved in water. While somewhat similar to aqueous glycol coolant formulations in freezing protection performance, aqueous solutions of low molecular weight organic acids also exhibit improved heat-transfer properties, lower viscosities, low toxicity to humans and animals and low adverse environmental consequences. Certain formate and acetate based fluids have known applications as heat-exchange fluids and airport runway deicing fluids. For example, U.S. Pat. No. 5,104,562 describes a coolant composition containing potassium formate and potassium acetate.
Any aqueous heat transfer fluid/freezing point depressant combination, including non-glycol based formulations, must also include corrosion inhibitors to reduce and control corrosion of the metal surfaces in cooling systems exposed to the fluid. Corrosion control in heat-exchange/cooling systems is highly desirable to mitigate the two principal adverse effects of metal corrosion. (1) deterioration of the metal components either by uniform wastage or localized attack (pitting, crevice corrosion) and, (2) the production of insoluble corrosion products that tend to foul cooling system parts and impede heat transfer by deposition of corrosion byproducts on heat exchange surfaces. These types of problems are addressed, for example in U.S. Pat. No. 6,689,289 which describes corrosion inhibiting, aqueous solutions of organic carboxylates having low eutectic temperatures. In particulars the '289 patent discloses aqueous solutions of low carbon (C1-C2) carboxylic acid salts, in combination with higher carbon (C3-C5) carboxylic acid salts, and C1 to C12 carboxylate corrosion inhibitors, which provide synergistically improved freezing and corrosion protection. Such improved freezing point and corrosion protection is demonstrated by adding one or more than one C6-C12 carboxylic acid salt corrosion inhibitors to (C1-C2)-(C3-C5) carboxylic acid salt freezing point depressants. It is also disclosed in the '289 patent that higher carbon carboxylates (C12-C16) add to the corrosion protection, but that the solubility thereof in the salt solutions is very limited. Such limited solubility reduces the degree of additional corrosion protection that could otherwise be obtained with use of the higher carbon carboxylates. It is also known in the art that the solubility of C6 to C16 carboxylate corrosion inhibitor combinations in aqueous C1-C2 freezing point depressant solutions is reduced, thereby limiting the corrosion protection afforded in such solutions.
Improved corrosion protection afforded by the higher carbon carboxylate corrosion inhibitors (C6-C16) and other inhibitor combinations is very important in heat-exchange/coolant applications where corrosion protection is critical, such as in the thermal management systems of internal combustion engines, industrial heat exchange systems, refrigeration and cooling systems, cooling towers, and any other thermal management system that must operate efficiently in a broad range of ambient temperatures. Additionally, in light of the interest in making non-toxic and environmentally benign heat exchange/coolant compositions available in the marketplace for use in the noted applications, to name a few, there is a need for new, non-glycol based heat exchange fluid compositions that exhibit a high specific heat, high thermal conductivity, low viscosity and that remain liquid over a wide range of temperatures. Heat exchange fluid compositions that meet this need can be provided and made more readily available if the solubility/stability of higher carbon carboxylate corrosion inhibitors can be improved. Accordingly, the object of the present invention is solve the aforementioned a solubility/stability problem and provide a non-glycol coolant/antifreeze composition that meets this need and that provides improved corrosion and freezing point protection over the prior art.